A vehicle may be equipped with an electric power steering system. The electric power steering system uses an electric motor to assist the driver of the vehicle in turning the wheels of the vehicle. Sensors detect the angular position and torque of the steering shaft and/or steering wheel, as well as the current operating conditions of the vehicle, and a steering microcontroller commands an assistive torque via the motor, to decrease the amount of torque that the driver must apply to turn the steering wheel, and thereby turn the wheels of the vehicle. The steering microcontroller is calibrated such that a varied amount of assistive torque is applied with the sensed operating conditions of the vehicle and thereby provides a desired steering feel to the vehicle operator.
One component of steering feel is hysteresis feel. Hysteresis feel can be described as the differences in driver applied steering torque required when steering the steering wheel away from center and returning the steering wheel to center, where center corresponds to a steering angle of zero. Hysteresis feel is the combined result of steering friction, steering inertia effects, and steering wheel velocity damping, also referred to herein as viscous damping. At low steering velocities, for example, at steering wheel velocities less than 45 degrees per second, friction is the main source of hysteresis. As steering velocities approach higher magnitudes, for example, as steering velocities approach 90 degrees per second, damping and inertia begin to add significant amounts to hysteresis feel. While generally friction is viewed as a negative for steering system components, some level of overall system hysteresis is required throughout the driving regime to provide a natural steering feel. With too little hysteresis the steering has a tendency to feel like a rotational spring. Inadequate hysteresis also limits how much steering effort progression is acceptable, e.g., the amount of driver steering torque required to maintain a given steering angle may be too high.
Calibrating the electric power steering system to provide the desired level of system hysteresis, including viscous damping, through the driving regime, is typically done during vehicle development based on subjective assessment due to the difficulty in mathematically modelling electric power steering systems at a sufficient level of accuracy to quantify these characteristics. Additionally, published objective steering metrics, predominantly based on sinusoidal steering inputs such as on-center and pulse steer tests, do not quantify all key steering feel characteristics, including, for example, viscous damping steering feel. Accordingly, subjective evaluation is used for the development of viscous damping steering feel, where this subjective methodology is disadvantaged by increased time for assessment, the prerequisite building of expensive physical testing vehicles, the limitations in the scope of conditions which can be assessed in a limited number of testing vehicles relative to modeled systems, and relatively less repeatability as compared with testing conducted in a laboratory environment and using a modeled system.